1. Field of the Invention
The present invention relates to a display, such as a liquid crystal display (LCD) device, a plasma display panel (PDP), an inorganic or organic electroluminescence (EL) display device, a light emitting diode (LED) display device, a fluorescence display tube, a field emission display device, an electrophoretic display device, an electrochromic display device, a cathode ray tube (CRT) display device, or the like.
2. Description of the Prior Art
The flat panel display (hereafter, referred to as “FPD”), such as a liquid crystal display, a plasma display, an EL display, or the like, has pixels arranged in a matrix on a substrate made of glass, plastic, semiconductor, or the like, and optically controls the pixels according to an external electric signal to display images. In general, each pixel includes sub-pixels of three primary colors (red, green, and blue). Each sub-pixel is the minimum unit of display. These sub-pixels are separately controlled by different signals.
In the FPD, the size of a pixel is determined by the size and resolution of the display. For example, in a 37-inch diagonal WXGA (Wide eXtended Graphics Array) FPD having the resolution of 1366×768, the size of one pixel is 600 μm×600 μm. The shape of a pixel is not necessarily a square depending on the specifications of a display but is a square in general. In the case where the size of one pixel is 600 μm×600 μm and one pixel is formed by stripe-shaped sub-pixels of R(red), G(green) and B (blue), the size of one sub-pixel is 200 μm (a ⅓ of the pixel width)×600 μm.
FIG. 13 is a general plan view of a commonly-employed pixel. The pixel 101 includes a red sub-pixel 102, a green sub-pixel 103, and a blue sub-pixel 104. The red sub-pixel 102, the green sub-pixel 103, and the blue sub-pixel 104 are electrically connected to a signal line 105 for the red sub-pixel, a signal line 106 for the green sub-pixel, a signal line 107 for the blue sub-pixel, respectively. In many liquid crystal display devices and organic EL displays, a signal line is connected to a sub-pixel through a thin film transistor (TFT), a diode, or the like. It should be noted that, in a duty-driven display device, such as a plasma display, or the like, a signal line itself functions as a sub-pixel. The red sub-pixel 102, the green sub-pixel 103, and the blue sub-pixel 104 are driven by signals supplied through the corresponding signal lines to carry out matrix display.
In general, a human eye has high sensitivity to green, and the ratio of lightness of the primary colors (red, green, blue) is approximately 5:12:2. In the stripe arrangement, sub-pixels of the same color are aligned in the vertical direction. Thus, when the color of white is displayed over the entire screen, vertical lines of yellow, which is the combination of red and green, and vertical lines of blue periodically occur. These lines are visually perceived as a vertical stripe pattern. Even with other display examples, such as a human face, plain (patternless) wallpaper, or the like, vertical stripes (vertical lines) occur over the entire screen, resulting in deteriorated display quality.
This phenomenon is now described in detail with reference to FIG. 1. FIG. 1 is a so-called Campbell-Robson CSF Chart. In this chart, the horizontal axis denotes the resolution of the stripes, and the vertical axis denotes the brightness ratio of the stripes (contrast). A pattern of light and shade of the stripes is modeled as a wave where the horizontal axis is the spatial frequency. The spatial frequency is represented by the number of pairs of black/white lines and spaces which exist in a view field of 1°. The unit of the spatial frequency is “cycle/degree”. The brightness ratio of the stripes (contrast) is represented by a gray scale of 8 bits on a display. Specifically, the 128th level (contrast=1) of the gray scale corresponds to the top of the Campbell-Robson CSF Chart of FIG. 1, and the 0th and 255th levels (contrast is about 500 to 1000) correspond to the bottom of the chart.
FIG. 2 is a graph showing a resolution limit curve measured using a Campbell-Robson CSF Chart. The vertical and horizontal axis are the same as those of the Campbell-Robson CSF Chart of FIG. 1. The inside of the curve of FIG. 2 is a region where the vertical stripes are seen. As seen from FIG. 2, the visibility of the stripe pattern of sub-pixels depends on the distance between a viewer and a display device, the size of sub-pixels and the contrast.
The contrast is the ratio of the intensity of blue to the sum of the intensities of green and red and therefore can be theoretically calculated. For example, in the case of a liquid crystal display device, the ratio of the transmittance of a color filter can be used as a substitute for the ratio of the color intensity. In a typical color filter, the transmittance of blue part is 8.7%, the transmittance of green part is 57%, and the transmittance of red part is 22%. Based on a calculation with these values, the transmittance ratio of the color filter is (57+22)/8.7=9.1. That is, the contrast is about 9. The transmittance is different among color filters of different types. In the case of an emission display, such as a PDP, or the like, the brightness is measured instead of the transmittance. However, the contrast is always about 9 so long as the display device is based on the RGB primary colors. Therefore, the stripe pattern is more conspicuous as the distance between a viewer and the display device is shorter and the sub-pixel size is larger, i.e., as the resolution of the display device decreases.
Even with sub-pixel arrangements other than the pixel arrangement of the three primary colors (red, green and blue), a stripe caused by a sub-pixel of a color having a higher spectral luminous factor (hereinafter, “luminosity factor”) and a sub-pixel of a color having a lower luminosity factor based on the same mechanism is visually perceived. For example, in the case of a pixel arrangement of three colors, cyan, magenta and yellow, a stripe is visually perceived between a yellow sub-pixel having the highest luminosity factor and a magenta sub-pixel having the lowest luminosity factor. In the case of a pixel arrangement of four colors, red, green, blue and white, a stripe is visually perceived between a white sub-pixel having the highest luminosity factor and a blue sub-pixel having the lowest luminosity factor. In the case of a pixel arrangement of five colors, red, green, blue, cyan and yellow, a stripe is visually perceived between a green sub-pixel having the highest luminosity factor and a blue sub-pixel having the lowest luminosity factor.
In the case where the FPD is used for a television display, an appropriate distance between a viewer and the display device is three times the diagonal size of the screen as in a conventional CRT television display. However, in the case of a television monitor for personal use which is also used as a monitor of a PC (personal computer), the viewing distance between a viewer and the display device becomes relatively short. For example, the viewing distance is equal to or shorter than the diagonal size of the screen in many cases. Further, in the television display for personal use, higher brightness is required than in conventional PC monitors. Thus, in the case where the viewing distance is set according to the purpose of the display device, the display quality is deteriorated by the stripe pattern unless the resolution is equal to or higher than a certain level.
Specifically, as seen from FIG. 2, when the contrast is 9, the resolution limit is about 13 cycle/degree. When this resolution limit is applied to a case where the distance between a viewer and the display device is 50 cm, the pixel size (pitch) is about 400 μm. If the pitch of pixels is larger than 400 μm, vertical lines are perceived. For example, in the case of a 50-cm (19.7-inch) diagonal VGA (Video Graphics Array) class display having the resolution of 640×480, the size (pitch) of one pixel is 680 μm. Thus, vertical stripes are visually perceived, and the display quality is deteriorated.
Occurrence of the stripes due to the stripe arrangement is modified to some extent by using a delta arrangement, a mosaic arrangement, or a square arrangement disclosed in Japanese Unexamined Patent Publication No. 6-102503. However, in these examples, stripes still occur as horizontal stripes or diagonal stripes. That is, the problem is not thoroughly overcome. Further, in the case of an image with clear edges, such as display of text, a vector image, or the like, the edges are displayed in unintended colors, and such edges of the unintended colors are undesirable in personal computers and computer graphic applications.
Japanese Unexamined Patent Publication No. 2001-272689 discloses a liquid crystal display device with a stripe color filter including color filter segments of n colors (n is an integer equal to or greater than 2) wherein color filter segments of the same color are aligned in the vertical direction, sets of m color filter segments of the same color (m is an integer equal to or greater than 2) are sequentially aligned in the horizontal direction, and one pixel is formed by n×m×1 sub-pixels (1 is a natural number). However, it is not preferable that the display resolution is equal to or higher than the resolution of an input signal because the cost of peripheral circuits, such as a driver circuit, a controller, etc., increases. Further, in the case where a signal having a resolution equal to or lower than the resolution of a display device is input to the display device, for example, in the case where a signal having the resolution of VGA is input to a XGA (eXtended Graphics Array) display panel, it is necessary to increase the resolution of the signal by image processing, and accordingly, an additional circuit is required.
Candice Hellen Brown Elliott, “Reducing Pixel Count without Reducing Image Quality”, Information Display, U.S.A, The Society for Information Display, December 1999, Vol. 15, No. 12, pp. 22-25, discloses a checkerboard pixel arrangement wherein a blue pixel is placed at the center. By employing this arrangement, at least vertical stripes are not observed. However, even when the checkerboard arrangement is employed without changing the resolution, a mixed stripe pattern including vertical stripes, horizontal stripes and diagonal stripes is observed because the spatial frequency is not changed. Thus, the above problem is not thoroughly solved by the checkerboard arrangement.